Variations in metal concentrations in a heavily polluted stream receiving acidic leachates from an old mine tailings deposit are analysed from a hydrological perspective. From an extensive data material, collected during three years, the variations in concentrations of four metals (copper, zinc, cadmium and lead) are discussed. The deposit is the principal source for these metals in the watershed. The variation in metal concentrations in the vicinity of the deposit can to a large extent be explained by simple dilution of contaminated water with uncontaminated water from the surrounding area. Further downstream the deposit, other processes become increasingly important for the metal concentrations in the water bodies.